1. Field of the Invention
The present invention relates to an optical scanning apparatus used for a copying machine and a laser printer.
2. Description of the Related Art
There have hitherto been widely utilized a digital copying machine and a printer, wherein electrically-charged photosensitive bodies are scanned by and exposed to light beams modulated corresponding to image information, electrostatic latent images are thus formed, and the images are obtained through electrophotographic processes such as developing, transferring and fixing.
Similarly, a full-color copying machine and a color printer are widely utilized, wherein image signals corresponding to yellow (Y), magenta (M), cyan (C) and black (K) undergo charging, exposing and developing, and a full-color image is formed by superposing and transferring these color images.
Over recent years, this type of full-color image forming apparatus has utilized a widespread so-called tandem system, wherein image forming portions corresponding to the developing colors (Y, M, C, K) are tandemly arranged, and the full-color image is formed via one path in a way that sequentially superposes the transferred images.
One example of the tandem system is a multi-beam scan apparatus (refer to Japanese Patent Application Laid-Open No. H11-119131), wherein a plurality of light beams incident on a deflection mirror side-by-side in a sub-scanning direction are deflected by mirror surfaces of the deflection mirror to be rotated. Then, the plurality of light beams scan on a plurality of scanned surfaces in a main-scanning direction. This optical scanning apparatus employs a so-called under-field optical system having a characteristic that a width, in the main-scanning direction, of the light beams incident on the deflection mirror is set narrower than a width, in the main-scanning direction, of one surface of the deflection mirror.
In the optical scanning apparatus discussed in Japanese Patent Application Laid-Open No. H11-119131, the plurality of light beams scan on the plurality of scanned surfaces in the mains-canning direction through one deflection mirror. Therefore, incidence optical elements making the light beams incident on the deflection mirror are required to be disposed side by side in the sub-scanning direction (vertical direction). The under-field optical system, however, if the incidence optical elements disposed side by side in the sub-scanning direction are not laid out with high accuracy, comes to have such a problem that positions in which to depict the scanned surfaces in the main-scanning direction deviate between the plural light beams.
If the incidence optical elements are disposed in close proximity for downsizing the apparatus, it is difficult to form bearing surfaces mounted with the incidence optical elements in the sub-scanning direction. Such being the case, if an angle made by the plurality of optical systems is set large at a long distance, such a problem arises that the apparatus gets upsized.
By the way, improvement of a recording speed (the number of output sheets per unit time) has been increasingly demanded of the image forming apparatus over recent years. In the printer and the copying machine, the increase in the number of output sheets per unit time must involve increasing a scan speed of the light beams on the photosensitive drums. A scan speed increasing method is exemplified by a method of increasing a rotating speed of a rotary polygon mirror and a method of providing a multi-beam system using a plurality of light sources. An over-field type optical scanning apparatus capable of obtaining an increased number of reflection surfaces while restraining a diameter of the rotary polygon mirror is known as one of the scan speed increasing techniques. The over-field type optical scanning apparatus has a characteristic that the width, in the main-scanning direction, of the light beams incident on the rotary polygon mirror is larger than the width, in the main-scanning direction, of one surface of the rotary polygon mirror.
One of the over-field type optical scanning apparatuses is an apparatus including lenses each having refracting power in only the main-scanning direction, wherein the width, in the main-scanning direction, of the light beams emitted from the laser light source is set large (refer to Japanese Patent Application Laid-Open No. 2004-020607).
If the construction that the plurality of light beams emitted from the plurality of light sources arranged side by side with the deviation in the sub-scanning direction scan on the plurality of scanned surfaces in the main-scanning direction through the single deflection mirror is applied to the over-field optical system, however, the optical elements provided between the plurality of light sources and the deflection mirror are required to be positioned with high accuracy in the main-scanning direction between the optical elements (e.g., cylindrical lenses) each having the same function. if not done so, when the plurality of light beams scan on the scanned surfaces in the main-scanning direction, distributions of light quantities used by the light beams differ from each other, and an image quality might decline.